Over the last three decades, materials for rewritable media have been receiving an increasing amount of interest in the scientific community and the industry. Since the discovery of chalcogenide film materials in 1968, materials have developed into a complex technology which find uses in many important applications, including electrical write and erase non-volatile memory devices, cognitive semiconductors, and rewritable optical discs.
Recent research efforts have been focussed on the development of non-volatile phase change random access memory and rewritable discs with high storage capacities and fast read-write speeds. Examples of such rewritable media that are now widely in use include rewritable compact discs (CD-RW), rewritable digital video discs (DVD-RAM, DVD-RW, DVD+RW) and rewritable blue laser optical disc (Blu-ray, HD-DVD).
Phase change media utilise phase change materials as recording media. The media is thermally written and optically read. Data is written and erased by inducing a change in phase of the material between crystalline phase and the amorphous phase. A laser beam is typically used to heat the material to bring about a change of phase. As each phase is associated with a different level of light reflectivity, namely, the crystalline phase is highly reflective, and the amorphous phase is lowly reflective, both lowly reflective mark and highly reflective background spots can be formed on the material to represent computer-readable data bits.
The use of rewritable material for optical discs was first reported in 1971 in which the recording media comprised chalcogenide phase-change composite material (Feinleib et al., Appl. Phys. Lett. Vol. 18 (1971) pg. 254). Since then, many attempts have been made to provide new materials for rewritable media in response to the increase in computer data size and the corresponding increase in the demand for larger storage capacity and faster read-write speeds.
For example, U.S. Pat. No. 5,709,978 discloses a phase change recording film for use in double sided optical discs in which the recording material comprises a phase change component such as Sb—Te—Ge and at least one lanthanide element and a transition metal. A high melting point component is precipitated in the recording film to coexist with a phase change component to prevent the recording film from flowing and segregating during recording and erasing.
Another type of rewritable media that is widely in use is magneto-optical (MO) media (used for example in Mini Discs). MO media operates based on both magnetic and optical storage device principles: writing is carried out magnetically after thermal treatment, and reading is carried out optically. Typically, a focussed laser beam is applied onto one side of the media in order to heat the MO material to its Curie point or compensation temperature, and thus render it susceptible to a magnetic field. A magnetic head positioned on the opposite side of the disc is then operated to record digital data onto the disk by altering the magneto-optical polarity of the heated area.
U.S. Pat. No. 6,132,524 discloses an example of a semiconductor magneto-optical material. The material comprises a semiconductor such as MsAs:GaAs in which fine magnetic particles are dispersed. The material exhibits magneto-optical effects at room temperature and can be used for signal processing and the fabrication of optical isolators and integrated circuits.
Composites comprising two or more kinds of material components and their use in recording media have been disclosed in U.S. Pat. No. 5,709,978. In carrying out the synthesis of the composites, material components of the composites are either combined and prepared from a simple mixture, or one material component is precipitated in the other material component. Such composites exhibit either magnetic or phase change properties individually, but is incapable of exhibiting both magnetic and phase change properties simultaneously.
Several studies into materials for optical discs have been carried out. In a study of the erasing process in optical rewritable discs, Shi et al. discloses the use of phase change material Ge2Sb2Te5 in optical rewritable discs and the dynamic crystallisation behaviour in relation to the erasure of data on such discs (Jpn. J. Appl. Phys. Vol. 42, Part 1, No. 2B, pp 841-847, 2003).
In another study, Sato et al. discloses a GaN-based ferromagnetic diluted magnetic semiconductor for semiconductor devices (Jpn. J. Appl. Phys. Vol. 40, Part 2, No. 5B, pp. L485-L487, 2001). In yet another study, Sun et al. (Appl. Phys. Lett. Vol. 82 No. 12 pp, 1902-1904, 2003.) discloses the use of FePt magnetic films sputtered on Cu.
Despite the developments that have taken place, limitations in current materials still exist. For example, the capacity and recording speed of rewritable optical media are presently limited by the laser diffraction limit and the crystallisation speed of the recording material. Therefore, continuing efforts are needed to provide new materials having new characteristics.
Accordingly, it is an object of the present invention to provide new materials that, for example, give rise to improved performance.